1. Field of the Invention
The invention relates to a reactor for producing polycrystalline silicon and also to a method for removing a silicon-containing deposit on a component of such a reactor.
2. Description of the Related Art
Polycrystalline silicon (polysilicon for short) serves as a starting material for producing monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone melting (FZ) process, and also for producing monocrystalline or multicrystalline silicon by means of various pulling and casting methods for producing solar cells for photovoltaics.
Polycrystalline silicon is generally produced by means of the Siemens process. In this process, in a bell-shaped reactor (“Siemens reactor”), substrates, usually thin filament rods (thin rods) of silicon, are heated by direct passage of current and a reaction gas containing hydrogen and one or more silicon-containing components is introduced.
Usually, as a silicon-containing component, trichlorosilane (SiHCl3, TCS) or a mixture of trichlorosilane with dichlorosilane (SiH2Cl2, DCS) and/or with tetrachlorosilane (SiCl4, STC) is used. The use of monosilane (SiH4) is also known.
The thin rods are usually embedded vertically in electrodes situated on the reactor base which provide the connection to the power supply. In each case two thin rods are coupled via a horizontal bridge (likewise made of silicon) and form a substrate for the silicon deposition. Owing to the bridge coupling, the typical U shape of the substrates is generated.
High-purity polysilicon is deposited on the heated thin rods and the horizontal bridge, as a result of which the diameter thereof grows with time.
Polycrystalline silicon granules, or polysilicon granules for short, are an alternative to the polysilicon produced in the Siemens process. Whereas the polysilicon in the Siemens process is produced as a cylindrical silicon rod, which, before further processing thereof, must be comminuted in a time-consuming and costly manner to form what is termed “chip poly” and may need to be purified again, polysilicon granules have bulk product properties and can be used directly as a raw material, e.g. for monocrystal generation for the photovoltaics and electronics industries.
Polysilicon granules are produced in a fluidized-bed reactor. This proceeds by fluidization of silicon particles by means of a gas flow in a fluidized bed, wherein the fluidized bed is heated to high temperatures via a heating device. By addition of a silicon-containing reaction gas, a pyrolysis reaction proceeds on the hot particle surfaces. In this process, elemental silicon is deposited on the silicon particles and the individual particles grow in diameter. By means of regular takeoff of particles that have grown and addition of smaller silicon particles as seed particles (“seed”), the method can be operated continuously with all of the associated advantages. Such deposition processes and devices therefore are known, for example, from U.S. Pat. No. 4,786,477 A.
It has been found that in these processes, silicon deposition occurs on the hot reactor parts, e.g. the reactor wall, internal parts and nozzles. Firstly, this concerns silicon deposited on the reactor parts. Secondly, this concerns silicon dust which grows on hot reactor parts.
US 20020102850 A1 discloses a method for avoiding or removing silicon deposition on reactant gas nozzles by continuous, discontinuous or controlled addition of HCl+inert gas (H2, N2, He, Ar) or inert gas H2.
US 20020081250 A1 describes a method in which separation or partial removal of the wall deposit by etching proceeds at operating temperature or close to the operating temperature of the fluidized-bed reactor using a halogen-containing gaseous etchant, such as hydrogen chloride, chlorine gas or silicon tetrachloride. However, such a process procedure is associated with increased operating costs.
U.S. Pat. No. 7,922,990 B2 claims a method in which, in a reactor having a hot surface, a reaction gas containing a gaseous silicon compound is deposited at a reaction temperature of 600 to 1100° C. as silicon metal on silicon particles which are fluidized in a fluidized bed by means of a fluidizing gas and are heated to the reaction temperature, and the particles furnished with the deposited silicon and also unreacted reaction gas and fluidizing gas are removed from the reactor, characterized in that on the surface of the reactor, a gas composition containing 99.5 to 95 mol % hydrogen and 0.5 to 5 mol % of the gaseous silicon compound is present, and the surface of the reactor has a temperature of 700 to 1400 degrees C., this temperature corresponding to the temperature of the silicon particles or being higher than the temperature of the silicon particles.
Via the combination of a high reactor surface temperature with a gas composition containing 99.5 to 95 mol % hydrogen and 0.5 to 5 mol % of the gaseous silicon compound, a reaction equilibrium may be established on the reactor surface, in which deposition of silicon onto the surface of the reactor virtually no longer takes place, and therefore a continuous process procedure is possible.
US 2008299291 A1 discloses how, by suitable choice of the process parameters of the the mean gas velocity in the two fluidized zones of the fluidized bed, the local gas velocities of the gases or gas mixtures at the exit of the nozzle systems, the pressure and the temperature of the fluidized bed, the placing of the nozzles relative to one another and relative to the fluidized bed containing wall and the residence time of the gases in the fluidized zones of the fluidized bed, a reaction course and therefore a concentration profile can be generated which ensures that the reaction gas is reacted to virtually the chemical equilibrium conversion rate before it reaches either the fluidized bed containing wall or the fluidized bed surface. As a result, wall deposition on the fluidized bed containing wall is reduced to a very low extent, which permits the unimpeded heating of the fluidized bed in the region of the reaction zone over a long period.
By the previously described processes, the silicon deposition on the reactor wall appears to be reducible by a suitable process procedure. However, this does not apply to deposits on other internal parts in the reactor or in the vicinity of the reactor, such as product takeoff tubes or off-gas tubes. In particular, this applies when the deposit consists of Si dust, which is discharged from the fluidized bed. Here also, in the prior art, removal of the wall deposit by etching is recommended.
U.S. Pat. No. 5,358,603 A discloses a method for etching a silicon deposit on a product takeoff tube, wherein the reactor is shut down, and the silicon deposit is heated and then etched using a mineral acid such as HCl. Primarily, the necessity to shut down the reactor makes this method complex and uneconomical. In addition, the operating media costs increase.
A particular problem is the fact that during relatively long operation of a fluidized-bed reactor, owing to deposit formation, the heat transfer in the off-gas heat exchanger is impaired. The off-gas can no longer be sufficiently cooled. In CVD Siemens reactors and the off-gas (Liebig) tubes thereof, those skilled in the art are also confronted with similar problems.
The problems described yielded the objective of the invention.